Electron beam amplifier tube with mismatched circuit sever

ABSTRACT

An electron beam amplifier tube has a traveling wave output circuit electrically isolated from the signal input circuit so that the output circuit is excited only by beam electron current. The upstream end of the output circuit is terminated with an impedance which reflects a substantial part of the backward wave energy flowing toward the upstream end. The phase and amplitude of the reflection may be chosen to reinforce the beam-circuit interaction, thereby improving tube efficiency.

United States Patent 11 1 Butwell et al.

[ 1 Dec. 2, 1975 l l ELECTRON BEAM AMPLIFIER TUBE WITH MISMATCHEDCIRCUIT SEVER [75] Inventors: Robert J. Butwell, Cupertino;

Gordon T. Hunter, Mountain View,

both of Calif.

[73] Assignee: Varian Associates, Palo Alto, Calif.

[22] Filed: Nov. 4, 1974 [21] Appl. No.: 520,524

[52] U.S. c1. 315/3.6; 315/35; 315/393; 330/43; 331/82 [51] Int. C1.H01J 25/34 [581 Field of Search 315/35, 3.6, 39.3, 39.51; 331/86, 88,91, 82; 330/43 {56] References Cited UNITED STATES PATENTS 3,123,7363/1964 Christoffers ct al. 3l5/3.6 3,365,607 1/1968 Ruetz ct al 315 353,538,377 11/1970 Slocum 315 36 3,576,460 4/l97l Harman 315/35 3,594,6057/1971 Blinn 3,668,544 6/1972 Lien 3.85:2,635 12/1974 Heynisch 315/35Primary E.raminerSaxfield Chatmon. Jr. Attorney, Agent, or Firm-StanleyZ. Cole; David R. Pressman; Richard B. Nelson [57] ABSTRACT An electronbeam amplifier tube has a traveling wave output circuit. electricallyisolated from the signal input circuit so that the output circuit isexcited only by beam electron current. The upstream end of the outputcircuit' is terminated with an impedance which reflects a substantialpart of the backward wave energy flowing toward the upstream end. Thephase and amplitude of the reflection may be chosen to reinforce thebeam-circuit interaction, thereby improving tube efficiency.

13 Claims, 8 Drawing Figures US atem Dec. 2, 1975 Sheet 2 of2 3,924,152

ELECTRON BEAM AMPLIFIER TUBE WITH MISMATCI-IED CIRCUIT SEVER Theinvention herein described was made in the course of or under a contractor subcontract thereunder, (or grant) with the Department of the AirForce.

FIELD OF THE INVENTION The invention relates to microwave amplifiertubes wherein a beam of electrons interacts with an electromagnetic waveon a slow-wave circuit which wave travels with velocity approximatelyequal to that of the electron beam. Such tubes include crossed fieldamplifiers, linear beam traveling wave tubes and hybrid traveling wavetubes with klystron-like driver sections known by the registeredtrademark Twystron". These types of tubes provide amplification over awide frequency range and can be designed for high power output and highefficiency.

In a traveling wave tube, the gain increases with the length of theslow-wave interaction circuit. When the gain becomes too high, e.g., dbor more, instabilities arise due to reflection of wave energy from theinevitable small transmission line impedance mismatches at the ends ofthe circuit. The reflected energy is reamplified, producing instability.

PRIOR ART In prior art tubes, the instability difficulty has beenreduced by severing the circuit at one or more points to form isolatedsections with reduced individual gain. Each upstream and downstreamsevered end is customarily terminated in a dummy load matched to itscharacteristic transmission line impedance. At the sever, the waveenergy on the circuit is dissipated in the load. The followingdownstream section of circuit is excited only by the alternating currentcomponent of the electron beam which carries signals only in the forwarddirection with no feedback.

The matched termination customarily provided at the upstream end of adownstream severed section was intended to prevent reflection of anywave traveling backward upstream in that section, because amplificationof the wave reflected downstream creates regeneration.

In the final output section of the circuit whose downstream end isconnected via a transmission line to a useful, external load,particularly large waves may be reflected backward by unavoidablemismatch of the useful load. It has therefore been common practice totry to obtain the best possible match to the dummy load at the upstreamend of the output circuit section, and it has been generally assumedthat a perfect match will give optimum performance.

It has been widely believed in the traveling wave tube art that the gainin the output section must be relatively high to provide efficientgeneration of output power. For example, Why a Circuit Sever AffectsTraveling Wave Tube Efficiency by A. W. Scott, IRE Transactions onElectron Devices, ED9, No. 1, page 35, teaches that in a typical highpower tube at least 26 db of growing-wave gain is needed. With thisregime of gain, a well matched sever termination is of course necessary.

As disclosed in US. Pat. No. 3,825,794 issued July 23, 1974 to Robert J.Butwell and assigned to the present assignee, it has recently beendiscovered that by providing a tightly bunched electron beam enteringthe output circuit, one can obtain good efficiency with less than 13 dbgain in the output section.

Some of the prior art circuit terminations have been used in severedcircuit traveling wave tubes and others in Twystron hybrid amplifiers asdescribed in US. Pat. No. 3,289,032 issued Nov. 29, 1966 to R. R. Rubertand R. L. Perry and assigned to the present assignee. In a hybrid tubethe signal is carried from the klystron cavities to the traveling waveoutput circuit only by the bunched beam, exactly as in a. severed TWT,so the requirements of the output circuit are the same for both types oftubes.

SUMMARY OF THE INVENTION The principal object of the present inventionis to provide a wide-band, electron beam, microwave ampli fier tube withimproved efficiency.

Another object is to provide a tube whose performance is insensitive towave reflections from its load.

Another object is to provide a tube of simplified construction.

These objects have been achieved in a beam tube with a short, travelingwave output circuit by providing at the upstream end of the circuit aterminating impedance which reflects a substantial portion of any waveenergy flowing backward toward the upstream end. With proper values ofthe phase and amplitude of the reflection the forward wave reflected bythe termination adds to the wave excited by the beam to enhance theinteraction efficiency.

In direct opposition to previously held belief that the output circuitmust have high gain and that it must have a matched termination,thepresent invention has produced increased efficiency with a short, lowgain circuit. Since the gain of the output circuit is low, changes inoutput load match produce smaller perturbations of the tube performance.Also, the short traveling wave circuit is simpler to fabricate andcheaper than previous long, high gain circuits.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic elevation view,partly in section, of a hybrid linear beam amplifier embodiment of theinvention.

FIG. 2 is an enlarged section of the output circuit of the amplifier ofFIG. 1.

FIG. 3A is an enlarged plan view section of the circuit of FIG. 2 alongthe line 3A3A.

FIG. 3B is an isometric view of a cavity of an alternative slow-wavecircuit.

FIG. 4 is an elevation section of a portion of another embodiment of theinvention corresponding to section 4 of FIG. 3A.

FIG. 5 is a view similar to FIG. 4 of still another embodiment.

FIG. 6 is a view similar to FIG. 4 of still another embodiment.

FIG. 7 is a schematic section of a crossed-field amplifier embodying theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention isapplicable to any electron beam amplifier tube wherein radio frequencypower is extracted from the beam by interaction with an electromagneticwave traveling at approximately the beam velocity on a severed circuit.Such tubes include linear beam traveling wave tubes and hybrid Twystronampli fiers as well as crossed-field amplifiers with injected beam. Forclarity of illustration the Twystron hybrid embodiment will be describedin detail.

FIG. 1 illustrates a hybrid Twystron amplifier. A cylindrical electronbeam is formed by an axially symmetric gun comprising a concavethermionic cathode 11, as of porous tungsten impregnated with bariumaluminate, and surrounded by a beam-focusing electrode 12 as ofaustenitic stainless steel operated at the same potential as cathodel 1. Gun 10 is mounted on a metallic base (not shown) sealed to aceramic insulating cylinder 21, as by brazing. The other end of cylinder21 is brazed to a steel cup 22 surrounding gun 10. A central aperture incup 22 is brazed to a copper anode electrode 23 containing a centralaxial aperture 24.

In operation, a negative voltage applied to gun 10 with respect to anode23 draws a converging electron beam through aperture 24. Surroundinganode 24 is an iron polepiece 25 for directing the field of asurrounding solenoid magnet (not shown) axially of the beam to focus itin an extended cylindrical outline.

The beam passes through a series of kylstron-type cavities 30, 31, as ofcopper, provided with reentrant annular drift tube sections 32 definingbeam interaction gaps 33. The first, upstream cavity is coupled via aconductive loop 34 to a coaxial transmission line 35 carrying the inputdrive signal for the amplifier. The signal is amplified by thewell-known klystron velocity modulation process.'The resonantfrequencies and Us of cavities 30, 31 are staggered as taught by theaforecited US. Pat. No. 3,289,032 to provide a tightly bunched beamovera wide frequency range such as 10% of the center frequency.

The bunched beam enters a traveling wave output circuit 40 comprising aseries of cavities 41 (FIG. 2) as of the cloverleaf type described inU.S. Pat. No. 3,233,139 issued Feb. 1, 1966 to Marvin Chodorow andassigned to the present assignee. The cavities are mutually coupled byradial slots 42 (FIG. 3A) to form a bandpass circuit propagating aforward-fundamental slow wave for interaction with the electron beam inthe well-known traveling wave tube fashion. Toward the output end of thecircuit the axial heights of the cavities are tapered to smaller valuesto reduce the circuit wave velocity to match the reducing velocity ofthe electrons as kinetic energy is extracted from them. The taperingprinciple is described in U.S. Pat. No. 3,374,390 issued Mar. 19, 1968to J. A. Ruetz, W. H. Yocom and Rene M. Rogers and assigned to thepresent assignee.

The final cavity 43 is coupled through an iris aperture 44 to a reducedheight rectangular output waveguide 45 which is matched by a taperingheight section 46 to a standard height output waveguide 47. interposedbetween two sections of guide 47 is a short section of cylindrical guide48 sealed by a ceramic vacuum window 49 as described in U.S. Pat. No.2,958,834, issued Nov. 1, 1960 to R. S. Symons and A. E. Schoennauer andassigned to the present assignee.

After passing through traveling wave output circuit 40 the beam goesthrough a second magnetic polepiece 50 which terminates the axialfocusing field. The beam then expands and is collected on the innersurface 51 of a collector 52, as of copper, mounted on polepiece 50 viaa cylindrical ceramic insulator 53. Collector 52 is cooled by watercirculating through inlet and outlet pipes 54 and small enclosedchannels (not shown) in the copper collector walls.

When the tube is operated, the electron beam enters output travelingwave circuit 40 with the electrons gathered into periodic bunches by thevelocity modulation in the klystron section. The alternating spacecurrent component represented by the bunches induces rf circulatingcurrent in the circuit cavities. This in turn produces an rf voltagewave traveling with the beam which increases the bunching, so the signalis amplified. At the same time energy is transferred from the beamskinetic energy to the circuit wave which is carried to the usefuloutput.

The electromagnetic fields in any given cavity couple energy through thecoupling slots to its two adjacent cavities, upstream and downstream, inapproximately equal amounts. The downstream fields add in proper phaseto those generated in succeeding downstream cavities to produce thegrowing forward traveling wave. In traveling wave tubes where thecircuit is many wave lengths long and the gain per circuit period issmall, the upstream fields of the successive circuit periods add up in acancelling phase, so there is very little backward flow of power.However, in very high power tubes the gain per cavity may be as much as3 db. In this case the cancellation of backward waves is very imperfect.In the upstream cavities the backward wave power may sometimes evenexceed the forward wave and may be a sizeable fraction of the tubesoutput power.

Previous TWT practice and theory has been based on the belief that theupstream end of the circuit should be terminated in a matched impedanceso that any backward wave is absorbed at the circuit input. According tothe present invention, it is found that improved efficiency can beobtained when all or a substantial portion of the backward wave incidenton the upstream end of the output circuit is reflected downstream. Thereflection must be of a carefully selected phase and ampli-' tude suchthat the reflected wave adds to the forward waves generated in the firstfew cavities to produce the proper phase and amplitude of rf voltage toenhance the bunching process. The interaction is complex and notdefinable in terms of a simple analogy. Results can be predicted bydigital computer calculations leading to specific designs suitable for aparticular tube. According to the these calculations the generalizedconcept of a mismatched sever termination should provide improvedefficiency in a wide variety of tubes. Experimental tests on one tubeindicate improved efficiency from 44% with a matched termination to 51%with a beneficial mismatch.

Returning now to FIG. 2, the upstream cavity of traveling wave circuit40 is coupled via an iris aperture 61 to a waveguide 62 which tapersfrom the height of cavity 60 to a standard height. In this embodimentcircuit 40 is matched for backward traveling waves into guide 62. Thedesired wave reflection is caused by an impedance transformer 63 inguide 62. Any standard impedance transformer may be used. Transformer 63is a capactive iris for example. The amplitude of the reflection iscontrolled by the height of the iris and the phase by its distance fromcavity 60. More complex microwave transformers may be used to adjust thephase and amplitude of the reflection to be a desired function offrequency as is well-known in the art of microwave impedance matching.

Wave power not reflected by transformer 63 proceeds down guide 62 and isabsorbed by a matched load termination 64, for example, a block ofceramic loaded with carbon particles and tapered in height to provide abroadband match.

FIG. 3A illustrates a Cloverleaf cavity, as indicated in FIG. 2, withthe upper lid removed to show coupling slots 42. FIG. 3A also shows thecoupling of upstream cavity 41 to sever load waveguide 62 through irisaperture 61. The width of iris 61 is adjiisted to provide the correctcoupling to match the impedance of slow-wave circuit 40 to waveguide 62.

FIG. 3B shows an alternate cavity useful as an element of a travelingwave circuitaccording to the present invention. Here the cavity 41 is asimple cylinder with short drift tubes 70 projecting from each end wall71. Coupling to adjacent cavities is through iris slots 72. In thiscavity circuit the fundamental traveling wave is a backward wave, butdrift tubes 70 introduce space harmonic fields with a forward componentwhich can interact with the beam.

FIG. 4 shows a different embodiment of the invention as applied to thesame type of slow-wave circuit 40 as shown in FIGS. 1 and 2. In FIG. 4,waveguide 62 is ter minated in a short circuit 65 at a distance fromcavity coupling iris 61 selected to reflect backward wave energy intocavity 60 in the proper phase. The amplitude of the reflected wave iscontrolled by a button of lossy dielectric material 66, as of carbongrains dispersed in beryllium oxide ceramic, disposed within cavity 60to absorb some of the energy therein. In some embodiments a completereflection may be desired and lossy button 66 would then be omitted.

FIG. 5 illustrates another embodiment using a slowwave circuit 40similar to the circuit of FIGS. 1 and 2. In FIG. 5 the phase of thereflected wave is controlled by the resonant frequency of the firstcavity 60. By making the distance d between noses 72' different from thecorresponding distance d between noses 72 of the other resonators 41 ofcircuit 40, the resonant frequency of cavity 60' is set so that energyis reflected in the desired phase. The amplitude of the reflection isdetermined by the size of iris 61 which determines the amount ofbackward wave energy coupled into waveguide 62 and absorbed in load 64.In this embodiment the backward wave is not matched to the loadwaveguide but partially reflected at iris 61'.

FIG. 6 shows still another embodiment wherein both phase and amplitudeof the backward wave are controlled internally of first cavity 60. Theresonant frequency, determined by nose spacing d, determines the phase.and the cavity Q. determined by lossy button 66, determines theamplitude. In some embodiments button 66 may be omitted to producecomplete reflection.

FIG. 7 shows a highly schematic embodiment of the invention in aninjected-beam crossed field amplifier. Inside a vacuum envelope 70 is athermionic cathode 72 heated by a radiant heater 73. A beam of electrons71 is drawn from cathode 72 by an accelerating anode 74 at a positivepotential 86 with respect to cathode 72. A uniform magnetic field Bperpendicular to the cathode and anode surfaces directs the beamperpendicularly to its initial direction and away from the cathode. Thebeam enters an interaction region between a flat, extended soleelectrode 75 operated at a potential 85 negative to cathode 72 and anextended slow wave circuit 76, 77 operated at a potential 87 positive toaccelcrating anode 74. In the illustrated embodiment circuit 76, 77 isstructurally integral with vacuum envelope 70, the metallic parts ofwhich are operated at the dc potential of circuit 76, 77.

Electron beam 71 drifts lengthwise of sole electrode and circuit 76, 77under the influence of the crossed electric and magnetic fields. Thefield amplitudes are selected so that the average drift velocityapproximates thewave velocity on circuit 76, 77.

Circuit 76, 77 is severed into two parts 76 and 77 which are isolatedagainst wave energy transmission therebetween by anon-propagating seversection 78. The input circuitsection 76 is: matched at its upstream endto a waveguide 79 for introducing the input microwave signal. The signalis amplified by synchronous interaction with beam 71 At sever 78 circuit76 is matched to a load waveguide 80 terminated in a non-reflective load81. The rf signal is carried over to output circuit 77 by the accomponent of current on beam 71. The upstream end of circuit 77 iscoupled to a waveguide 82 provided with a wave-reflecting transformer 83and a matched load 84 to absorb non-reflected energy. Backward waveenergy on circuit 77 is reflected in guide 82 with a phase and amplitudeselected to enhance the interaction of the combined forward wave and theelectron beam. The forward wave is amplified in output circuit section77 and matched at the downstream end thereof into output waveguide 85for conduction to a useful load.

The several embodiments described to clarify the invention areillustrative only. Many other embodiments will be apparent to thoseskilled in the art. The invention is therefore intended to be limitedonly as described in the following claims and their legal equivalents.

What is claimed is:

1. An electron beam amplifier tube comprising; a vacuum envelope, meansfor forming and directing a beam of electrons, means for collecting saidelectrons, first circuit means for modulating said beam at a highfrequency, second circuit means downstream of said first circuit meansand isolated therefrom to prevent propagation of electromagnetic energytherebetween, said second circuit means comprising, a slow-wave circuitcapable of propagating a traveling wave in energyexchanging relationshipwith said beam and having an upstream end adjacent said first circuitmeans and a downstream end removed from said upstream end in thedirection of flow of said electrons,

an output transmission line matched to said downstream end of saidslow-wave circuit to couple into said transmission line a greater partof the wave energy flowing in said slow-wave circuit toward saiddownstream end, and terminating means at said upstream end of saidslow'wave circuit. the improvement wherein,

said terminating means comprises means for reflecting into a downstreamflowing wave a selected substantial portion of wave energy flowingupstream in said slow wave circuit toward said upstream end, thereflected wave having a phase with respect to said upstream waveselected to increase the effi' ciency of said tube over a wideoperational frequency range.

2. The apparatus of claim 1 wherein said reflecting means comprisesmeans for reflecting substantially all of said wave energy flowing insaid slow-wave circuit.

3. The apparatus of claim 1 wherein said reflecting means comprises atransmission line coupled to said upstream end of said slow-wave circuitand containing a wavereflective impedance discontinuity.

4. The apparatus of claim 3 wherein said transmission line furthercontains an attenuator.

5. The apparatus of claim 3 wherein said wavereflective discontinuity isa short circuit.

6. The apparatus of claim 1 wherein said slow-wave circuit is a seriesof mutually-coupled periodic elements.

7. The apparatus of claim 6 wherein said periodic elements are cavitieswith conductive walls.

8. The apparatus of claim 6 wherein said reflecting means comprises oneof said periodic elements with electrical characteristics different fromsucceeding said elements comprises energy-attenuating means.

1. An electron beam amplifier tube comprisiNg; a vacuum envelope, meansfor forming and directing a beam of electrons, means for collecting saidelectrons, first circuit means for modulating said beam at a highfrequency, second circuit means downstream of said first circuit meansand isolated therefrom to prevent propagation of electromagnetic energytherebetween, said second circuit means comprising, a slow-wave circuitcapable of propagating a traveling wave in energy-exchangingrelationship with said beam and having an upstream end adjacent saidfirst circuit means and a downstream end removed from said upstream endin the direction of flow of said electrons, an output transmission linematched to said downstream end of said slow-wave circuit to couple intosaid transmission line a greater part of the wave energy flowing in saidslow-wave circuit toward said downstream end, and terminating means atsaid upstream end of said slow-wave circuit, the improvement wherein,said terminating means comprises means for reflecting into a downstreamflowing wave a selected substantial portion of wave energy flowingupstream in said slow wave circuit toward said upstream end, thereflected wave having a phase with respect to said upstream waveselected to increase the efficiency of said tube over a wide operationalfrequency range.
 2. The apparatus of claim 1 wherein said reflectingmeans comprises means for reflecting substantially all of said waveenergy flowing in said slow-wave circuit.
 3. The apparatus of claim 1wherein said reflecting means comprises a transmission line coupled tosaid upstream end of said slow-wave circuit and containing awave-reflective impedance discontinuity.
 4. The apparatus of claim 3wherein said transmission line further contains an attenuator.
 5. Theapparatus of claim 3 wherein said wave-reflective discontinuity is ashort circuit.
 6. The apparatus of claim 1 wherein said slow-wavecircuit is a series of mutually-coupled periodic elements.
 7. Theapparatus of claim 6 wherein said periodic elements are cavities withconductive walls.
 8. The apparatus of claim 6 wherein said reflectingmeans comprises one of said periodic elements with electricalcharacteristics different from succeeding downstream elements.
 9. Theapparatus of claim 8 wherein said one of said elements has aself-resonant frequency different from said succeeding elements.
 10. Theapparatus of claim 8 wherein said one of said elements comprisesenergy-attenuating means.
 11. The apparatus of claim 9 wherein saidportion of said wave energy is at least one fourth.
 12. The apparatus ofclaim 6 wherein said reflective means comprises one of said periodicelements which is electromagnetically coupled only to succeedingdownstream elements.
 13. The apparatus of claim 12 wherein said one ofsaid elements comprises energy-attenuating means.